Physicians commonly prescribe supplemental oxygen to enrich respired air for those patients whose pulmonary systems cannot extract sufficient oxygen from ambient air. Systems to provide oxygen enrichment include a supply of oxygen, stored either as a cryogenic liquid or as a pressurized gas, and a metering device to regulate flow of oxygen to the patient. Such systems are frequently portable to permit patient mobility during treatment. The more sophisticated systems also include an inhalation sensor and dose dispensing apparatus which conserves oxygen by supplying it only while the patient is inhaling. Conservation of oxygen in this manner serves to significantly reduce the system weight as a smaller quantity of oxygen is needed for any given service time as compared to those systems employing a continuous flow of oxygen to the patient. Because of the physical limitations of breathing impaired patients it is essential that portable systems be kept as light and as small as possible.
Those systems which employ a dose dispensing apparatus to supply oxygen only while the patient is inhaling have come to be known as pulse dose devices. Typically these devices operate by sensing the beginning of an inspiration and delivering pulses or doses of oxygen at a relatively high rate beginning at the start of inspiration and lasting only for a small part of the inspiration period after which the rate is reduced to some lesser value or to zero. The sensors and control circuitry for such devices are ordinarily powered by electricity and require a current source such as a battery. Also, the valves used to control oxygen flow are usually electrically operated solenoid valves.
Pulse dose oxygen systems known in the prior art fall generally into two types; one type employing rate-time metering and the other type employing volumetric metering. An example of a rate-time metering device is that described in the Durkan et al U.S. Pat. No. 4,462,398. That device employs an electrically operated solenoid valve controlled by a timer circuit in line with a flow controlling orifice. A sensor, which may be a pressure-to-electric switch or a strain gage bridge circuit such as is employed in Honeywell's 170PC series pressure sensors, detects the onset of an inhalation. That causes the timer circuit to activate the solenoid valve for a pre-determined interval thereby delivering a sized dose of oxygen to the patient. Volumetric metering devices employ a chamber sized to release a single dose quantity of oxygen through a solenoid valve which is caused to open in response to a signal indicative of the onset of an inhalation. Examples of volumetric metering systems known in the prior art are shown in applicant's U.S. Pat. Nos. 4,705,034 and 5,005,570.
All portable pulse dose delivery systems require enough electrical storage capacity to supply the needs of the system to operate the sensor, the control system and the solenoid valves which control gas flow for that time period required to deplete the supply of oxygen carried by the system. It is common practice in the art to use batteries which are periodically recharged or replaced to supply the electrical storage capacity for the system. The battery capacity required adds significantly to the bulk and weight of a portable system.
The supply of oxygen for all portable pulse dose systems is stored under pressure to provide greater storage density and to provide the motive power for dispensing it. Oxygen when stored as a gas is normally pressurized to hundreds of pounds per square inch. When stored as a cryogenic liquid, oxygen is maintained at a pressure typically ranging from about 20 to 50 psig by a relief valve which vents normal evaporation from the storage tank at that pressure. Pressurized liquid is vaporized and superheated at storage pressure before being metered and delivered to the patient. The energy of the gas expanding between storage pressure and ambient atmospheric pressure is presently wasted except for that small amount needed to expel and deliver it.
With this background it may readily be appreciated that a reduction in size and weight of systems for supplying supplemental oxygen to ambulatory patients would provide substantial advantage.